Joint arthroplasty is a well known surgical procedure by which a diseased and/or damaged natural joint is replaced by a prosthetic joint. Joint arthroplasty is commonly performed for hips, knees, elbows, and other joints. The health and condition of the joint to be replaced dictate the type of prosthesis necessary to replace the natural joint. For example, in a total hip arthroplasty an acetabular cup may be implanted in the pelvis to replace the natural acetabulum.
To implant an acetabular cup, an acetabular cavity is reamed in the acetabulum. The reamed cavity generally conforms to an outer surface of the acetabular cup. The acetabular cup is then inserted into the formed cavity and is then further secured by mechanical means, such as one or more fixation screws. The acetabular cup is positioned in the pelvis at a relatively fixed orientation with respect to patient anatomy and should remain stable.
FIGS. 1-4 show one type of prior art acetabular cup 10 implanted in a patient's acetabulum 12. The cup 10 includes a hemispherical outer member 14 for interfacing to bone and a mating inner member 16 for providing an articulation surface for the ball 18 of a femoral component 20. The inner member 16 is formed from polyethylene, for example, to provide a durable, low friction interface for allowing the femoral component to move freely. Apertures 22 are formed in the outer member 14 of the cup to provide passageways for fixation screws 24 that secure the acetabular cup 10 to the bone.
While the apertures allow the fixation screws to penetrate bone, the screws must be inserted at locations corresponding to the apertures. In addition, the fixations screws must be introduced into the bone within a limited angular range to allow proper seating of the screw head in the aperture.
Further, while the implanted cup may initially be secured in position, movement of the implanted acetabular cup over time can contribute to erosion of the surrounding bone. One effect of such bone erosion is the loosening of the acetabular cup, allowing it to shift in position. More particularly, the implanted cup tends to subside into the surrounding bone so as to adversely affect conditions in the prosthetic joint. Typically, an implanted acetabular cup will subside several millimeters within a few years after implantation, which can result in one or more fixation screw impinging on the cup liner. Screw/liner contact can cause fretting of the liner and possibly catastrophic failure of the liner.
For example, as shown in the prior art prosthetic hip joint of FIG. 4, as the cup 10 subsides into the surrounding bone a head 26 of the fixation screw 24 begins to impinge upon the polyethylene inner cup member 16. The action of the screw 24 against the polyethylene inner member 16 as the joint is subjected to loads may cause wear debris to develop, which can ultimately result in osteolysis within the joint. In extreme cases, the screw 24 can fracture the inner member 16 causing catastrophic failure of the joint.
It would, therefore, be desirable to provide an implantable prosthesis system having features to optimize the positioning of fixation screws into bone, to minimize the effects of implant subsidence into the surrounding bone, and to increase safety margins.